People start saying Merry Xmas to everyone from now onwards.

As for whether or not you should say it to everyone, I wouldn't. It's not extremely offensive, but it's not very nice to presume that everyone celebrates Christmas. "Happy holidays" is considered much more neutral.

That's right :) People in Russia usually say it on 7th January only.

I say it on 7th January to some people :) С Рождеством!